Multiple beam cathode ray tubes are frequently used to display alphanumeric and/or other visual pattern information. Such tubes have greater bandwidth than single beam tubes, which enables them to display more information at suitable brightness than the single beam type.
Typically, the multiple beam tubes utilize a plurality of closely spaced electron beams which are arranged in a vertical column array. Accelerating means, focussing means and deflection means are disposed in or on the envelope of the cathode ray tube, and after being accelerated and focussed, the beams are deflected across the screen while repeatedly being turned on and off so as to form "dots" on the screen at respective scanning positions. In order to form the desired characters or other patterns, logic circuitry selectively controls each beam to be either on or off at each scanning position, and the resulting arrangement of "dots" forms the desired pattern.
One problem which has been encountered with multiple beam cathode ray tubes is the presence of off-axis aberrations. Since only one beam can be emitted along the axis of the tube, the remainder of the beams in a multiple beam tube are off-axis by varying amounts. The abberrations are caused by off-axis imperfections in the focussing and deflection fields, and the imperfections, and therefore, the aberrations, increase with distance from the axis.
In the conventional multiple beam tubes, the beams are emitted parallel to the axis and are accelerated in the same direction to the focussing means or lens, which changes the direction of the beams and causes them to converge towards a crossover point which is located in the funnel portion of the tube.
In accordance with this arrangement, the parallel beams are spaced from each other by a substantial distance, resulting in a relatively large maximum off-axis distance as the beams traverse the focussing means, and due to the fact that the beams do not cross until they are well into the funnel, a relatively large maximum off-axis distance again results as the converging beams traverse the deflection means. Actually, the magnetic deflection yoke is the component which introduces the largest aberration, and the distortion is most severe when a preferred large deflection angle, which permits the length of the tube to be minimized for a given screen size, is employed. The off-axis aberrations caused by the conventional components and arrangement described above prevent the beams from being focussed to desired locations on the screen, and have proven to be quite troublesome.
A possible expedient for reducing the maximum off-axis distance as the beams traverse the focussing and deflection means is the use of an additional lens. However, such an arrangement would necessarily increase the overall length of the cathode ray tube and thus is not desirable.
An approach disclosed in the prior art is the use of a curved cathode for emitting initially converging beams which may cross each other at a point near the deflection means. For example, Houston U.S. Pat. No. 3,778,659 and Miram et al. U.S. Pat. No. 3,843,902 show curved cathodes which emit converging electron beams. The problem with this approach is that curved cathodes are difficult to manufacture, and may increase the manufacturing and selling cost of the tubes.